The number of obese or overweight people in the developing world nearly quadrupled from 250 million to 904 million between 1980 and 2008, according to a study published this month by a UK think tank. Over the same period obesity rates in high-income countries increased by 1.7 times.

One third of all adults worldwide—1.46 billion people—are now overweight or obese, says the report from the Overseas Development Institute. But by 2008, more people were overweight and obese in developing countries (904 million) than in richer countries (557 million).

The developing world’s dramatic weight gain since 1980 is due primarily to two factors, says the report: richer diets and more sedentary lifestyles. More people in poorer countries are earning enough to move from diets built on cereals and tubers to diets rich in meat, fat and sugar, and now have increasingly stationary lives. That’s leading to rising global incidence of diseases like cancer, diabetes, heart disease and stroke.

“What has changed,” says the report, “is that the majority of the people who are overweight or obese today can be found in the developing, rather than the developed, world.”